Code was using the loop [which is looping over the selection] index as
an index for the lookup into the edges directly, but needs to be a
lookupinto the IndexMask.
Also renamed the variable (as used elsewhere) to make this more clear.
If accepted, would be nice to still get this into 3.2.
Maniphest Tasks: T98536
Differential Revision: https://developer.blender.org/D15089
7dd0258d4a